There are various kinds of electron emission, such as field electron emission, secondary electron emission, and photoelectron emission, in addition to thermionic emission. A cold cathode is a cathode for producing electron emission by field electron emission. In field electron emission, a strong electric field (109 V/m) is applied near to the surface of a substance in order to lower the potential barrier of the surface, thus producing electron emission by tunnel effect. The cathode is referred to as a cold cathode because it does not require heating as a hot cathode does.
The current-voltage characteristics of the cold cathode can be approximated by Fowler-Nordheim equation. The electron emitting portion has such a structure (needle-like, for example) that it has a large electric field concentration constant, allowing the application of a strong electric field while maintaining insulation. Earlier cold cathodes had a diode structure and were made by electrolytic polishing of a needle-like single crystal or whisker. In recent years, the micromachining technologies in the fields of integrated circuits and thin films have been applied to the manufacture of field emission-type electron sources (field emitter array) for emitting electrons in a high electric field so successfully that field emitter cold cathodes with extremely small structures are now being made. The field emission-type cold cathode of this type is the most basic electron emission device of all the major components of a micro triode electron tube or a micro electron gun. Due to the structure miniaturization, the cold cathode has such advantages that it can realize a higher current density as an electron source than the hot cathode, and that electron sources which are separated into micro regions can be formed.
Research and development of the field emission-type electron sources are being actively conducted, with the expectations that a field emission display (FED) utilizing the cold cathode can be applied to self-emitting flat panel displays.
Various materials are known for the field emission-type electron source used in FEDs. In order to obtain sufficient electron emission, electric field strength of 1000 V/μm in effective value is required. Thus, when conventional materials are used, the above-mentioned structure is utilized to obtain a large electric field concentration constant, so that values of the actual applied electric field strength can be on the order of 100 V/μm.
In recent years, it has been confirmed that carbon materials such as carbon nanotube can perform electron emission with very small electric field strengths and gaining attention as an electron emission material.
FIG. 1 is a cross sectional view of conventional FED. Numeral 17 designates a face plate, 18 a phosphor, 19 phosphorous emission, 20 a spacer, 21 a back plate, 22 a metal back, and 3 an emitter.
In the FED, as in a CRT, an accelerated electron 6 collides the phosphor 18 and causes it to produce the emission 19, by which an image can be displayed. The face plate 17 is coated with the phosphor 18. As the phosphor material, high-voltage types used for the CRT or the like are the mainstream for the purpose of ensuring luminance. In this case, an aluminum thin film (metal back) 22 is formed on the incident side of the electron beam 6 in order to prevent a chargeup of the phosphor and the ion burning and to increase luminance. The space between the face plate 17 and the back plate 21 is maintained at vacuum. Therefore, the spacers 20 are provided at certain intervals to support the atmospheric pressure and maintain the gap.
FIG. 2 shows cross sections of the structures of conventional emission-type electron sources. Numeral 2 designates a gate insulator film, 3 a field emitter (emitter), 4 a gate electrode, 5 a focusing electrode, 6 emitted electrons (electron trajectories), 7 an equipotential surface, 8 a gate insulator film, 11 an anode electrode, and 14 a cathode wiring.
In any of the emission-type electron sources, a protruding electron emitter 3 is formed on a semiconductor or metal substrate, as shown in FIG. 2(a). The gate electrode 4 is formed around the emitter in order to apply an electric field for drawing electrons.
The electrons emitted by the emitter by the application of a voltage to the drawing electrode travel toward the anode 11 formed above the emitter, as shown in FIG. 2(a).
In these cold-cathode field emission-type electron sources, a sufficiently high electric field is applied between the gate electrode and the emitter in order to produce electron emission. A positive voltage is applied to the anode to collect the emitted electrons. There is the problem, however, that the emitted electrons spread, as shown by the electron trajectories 6 of FIG. 2(a), because the anode-gate electric field is weaker than the gate-emitter electric field.
In the conventional cold-cathode field emission-type electron sources with the projecting electron emitter, the spread of electrons is suppressed by providing the focusing electrode 5 as shown in FIG. 2(b), as disclosed in JP Patent Publication (Kokai) No. 7-29484.
In JP Patent No. 2776353, the focusing electrode 5 is provided in the same plane as the gate electrode 4, as shown in FIG. 2(c), in order to suppress the spread of electrons. It further proposes providing focusing electrodes on a pixel-by-pixel basis.
JP Patent No. 2625366 proposes making the insulating film thinner near the projecting emitter and thicker at other locations in order to focus the electron beam, as shown in FIG. 4(a).
Recently, an electron source structure has been proposed in JP Patent Publication (Kokai) No. 2000-156147, for example, in which, in a field emission-type device comprised of an anode, gate, and emitter as shown in FIG. 2(d), electron emission is produced by an anode-emitter field and the electron beam is focused by a gate-emitter electric field. The area of an opening of the focusing electrode is proposed to be smaller than that of the bottom surface of the focusing electrode opening.
Further, JP Patent Publication (Kokai) No. 2000-243218 proposes a structure for a field emission-type device comprising an anode, gate and emitter, as shown in FIG. 2(e), in which the anode-gate electric field is stronger than the gate-emitter electric field so that a downwardly protruding equipotential surface can be formed to provide a focusing effect. In this case, the electrons from the emitter are drawn by the electric field from the anode.
When the emission of electrons is caused by the electric field from the anode, it is necessary to employ materials that can emit electrons with a low electron field, such as carbon nanotube as mentioned above, as the material for the field emission-type electron source.
These field emission-type electron sources according to the prior art have the following problems.
In the cold-cathode field emission-type electron source having a projecting electron emitting portion, the focusing electrode 5 is provided to prevent the diffusion of electrons, as shown in FIG. 2(b). As a result, the number of manufacturing steps increases and the structure of the device becomes complex.
In the case where, the case not necessarily involving a projecting emitter, a far larger electric field is applied between the gate electrode and the emitter than that at the surrounding areas to produce electron emission and the electrons are focused by a separate focusing electrode, the electrons that have large velocities in diffusing directions are focused, so that greater energy must be dispensed for focusing purposes, thus resulting in reduced efficiency.
Reasons for the poor efficiency are shown in FIG. 3. When a far larger electric field is applied between the gate electrode and the emitter than that at the surrounding areas in order to produce electron emission, the electrons first pass through upwardly protruding equipotential surfaces (diffusing effect) and then downwardly protruding equipotential surfaces (focusing effect), as shown in FIG. 3. Electrons 6 move while they are accelerated in directions perpendicular to equipotential surfaces 7. When an electron passes through an electrostatic lens in which upwardly and downwardly protruding equipotential surfaces of identical shape are formed, as shown in FIG. 3(a), the electrons pass through the upwardly protruding equipotential surfaces (diffusing effect) when its initial velocity is slow. Accordingly, in this stage, the electrons receive lateral-direction forces (diffusing effect) for a longer time, so that the diffusing effect is greater. On the other hand, when the electrons pass through the downwardly protruding equipotential surfaces (focusing effect), the electrons have greater velocity and are therefore influenced by the lateral (focusing-direction) force for a shorter time, so that the focusing effect is less. As a result, the electrostatic lens as a whole operates as a diffusing lens (d in<d out).
If the diffusion is to be reduced (d in=d out), the curvature of the downwardly protruding equipotential surfaces (focusing effect) must be made smaller than that of the upwardly protruding equipotential surfaces (diffusing effect), as shown in FIG. 3(b). This requires greater energy because a greater potential difference must be created in this region.
In reality, the electric field strength between the gate electrode and the emitter is so great that the upwardly protruding equipotential surfaces become denser. As a result, the electrons possess greater velocities in the diffusing directions, requiring still greater energy for focusing purposes.
In JP Patent No. 2776353, as shown in FIG. 2(c), the application of a negative potential to the focusing electrode 5 results in a large potential difference between the focusing electrode and the drawing electrode to which a positive potential is applied, increasing the load on the drive circuit.
In JP Patent No. 2625366, as shown in FIG. 4(a), the electrons first pass through upwardly protruding equipotential surfaces (diffusing effect) and then downwardly protruding equipotential surfaces (focusing effect). This results in a reduced focusing effect for the reason mentioned above. Further, if this prior art technique were to be applied to a FED, the potential relationship would be as shown in FIG. 4(b), in which the focusing effect is hardly obtained. In order to obtain the focusing effect, the thickness of a thicker portion of the insulating film must be made far greater than that of a thinner portion, which is very difficult.
In the apparatus according to JP Patent Publication (Kokai) No. 2000-156147, the gate electrode is used for focusing the electron beam and, as shown in FIG. 2(d), the area of the gate electrode opening is made larger than that of the gate opening bottom surface. This makes it difficult to completely suppress the electric field from the anode, and also complicates the manufacturing steps.
In the apparatus known from JP Patent Publication (Kokai) No. 2000-243218, as shown in FIG. 2(e), the gate electrode doubles as the electron-beam focusing electrode. While this can simplify the manufacturing process, the spot size of the electron beam emitted by the electron source on the anode surface (phosphor surface) is determined by the ratio between the anode-emitter electric field strength Ea and the gate-emitter electric field strength Eg. In the following, the relationship between the electric field strengths Ea and Eg will be discussed.
When the value of Eg is too small as compared with the value of Ea, the electron-beam focusing effect of the gate electrode would be too strong, so that the spot size would be larger than the gate opening diameter before the beam arrives at the anode surface after having been focused in front of the anode surface. This would lead to crosstalk and thus reduce the image quality.
In an FED utilizing the high-voltage type phosphor mentioned above, there is a lower limit value for the anode potential for ensuring luminance (for ensuring phosphor emission efficiency, metal-backed layer transmission). The distance between the face plate and the back plate should desirably be small from the viewpoint of the shape of the spacer (aspect ratio) and the prevention of the diffusion of electron beam. The distance, however, should be large from the viewpoint of maintenance of the insulator withstand voltage. In view of such a tradeoff, currently the lower limit value of the anode voltage is set to be on the order of 5 kV, and the distance between the face plate and the backplate is set to be on the order of 1 mm, and the anode-emitter electric field strength Ea of the order of 5 V/μm is used.
When carbon nanotube is used as the electron emission material, necessary current densities for the FED can be obtained with the gate-emitter field strength Eg on the order of 2 V/μm.
FIG. 5 shows a cross sectional view illustrating how the electron beam spread in the configuration of FIG. 2(e).
As shown in FIG. 5, the spot size on the anode surface becomes larger than the gate opening diameter, and the condition is such that Eg is too small with respect to Ea. When Ea is 5 V/μm in the above-mentioned configuration, the condition Eg≧3 V/μm is necessary so that the spot size on the anode surface is to be no larger than the gate opening diameter.
The spot size can be reduced by sacrificing the electron emission characteristics and increasing the value of Eg. This, however, will result in applying a high electric field in a minute region between the gate and the emitter, which increases the danger of dielectric breakdown due to surface creepage on the insulator film, for example.
The present applicant has filed JP Patent Application No. 11-214976 (1999) concerning a cold-cathode electron source in which the emitter is divided on a pixel-by-pixel (or subpixel-by-subpixel) basis and in which a plurality of gate openings are provided so that the influence of an increase in the spot size at the center of the pixels can be prevented.
FIG. 6 shows a cross-sectional view illustrating how the electron beam spread in the above-mentioned divided gate/emitter configuration. In FIG. 6, numeral 1 designates a substrate, 2 a gate insulating film, 3 an emitter, 4 a gate electrode, 6 emitted electrons, 14 a cathode wiring, and 15 a ballast resistor layer.
As described in JP Patent Application No. 11-214976 (1999), the influence of spot size increases at the center of the pixels can be prevented by dividing the emitter on a pixel-by-pixel (or subpixel-by-subpixel) basis and providing a plurality of gate openings. However, as shown in FIG. 6, the spots of the electron beams emitted via the gate openings formed on the periphery of the pixels still spread beyond the pixel region, similarly causing crosstalk.
In view of these problems of the prior art, it is the object of the invention to provide at low cost an electron emission device and a FED comprising a cold-cathode electron source that has a high utilization efficiency of electron beam and that is capable of suppressing the spread of electron beam.